1. Field of the Invention
The present invention relates to a method for drawing an optical fiber preform to manufacture an optical fiber, in a manufacturing process of an optical fiber.
2. Description of the Related Art
FIG. 15 is a schematic configuration view showing a general apparatus for manufacturing an optical fiber. A method for manufacturing an optical fiber using this manufacturing apparatus includes the following steps.
(1) Inserting an optical fiber preform 101 which is formed from a glass rod which becomes a source of an optical fiber into a heating furnace 102. Then, a tip of the optical fiber preform 101 is heated and melted at a temperature of about 2000° C. by a heater 102a, and a bare optical fiber 103 is drawn out downward from the heating furnace 102.
(2) Cooling the drawn-out bare optical fiber 103 by a cooling device 104 provided under the heating furnace 102. The cooling device 104 includes a vertically long cooling cylinder. A cooling gas (helium gas or the like) is supplied to the inside of this cooling cylinder from a side portion of the cooling cylinder. In FIG. 15, a flow 110 of the cooling gas indicated by an arrow towards an upper portion and a lower portion within the cooling cylinder, and the bare optical fiber 103 drawn out from the heating furnace 102 is sufficiently cooled by this cooling gas until the bare optical fiber 103 reaches a temperature capable of coating.
(3) Coating a coating resin on the periphery of the bare optical fiber 103 for the purpose of protection of the surface of an optical fiber glass, thereby forming a protective covering layer to obtain an optical fiber 107. First, the coating resin is coated on the cooled bare optical fiber 103 by a coating device 106. Next, this coating resin is heat-cured or ultraviolet-cured by the curing device 108, and is formed as a protective covering layer. Generally, this protective covering layer is formed with a two-layer structure. Coating is performed using a material with a low Young's modulus for an inner layer and using a material with a high Young's modulus for an outer layer.
(4) Winding the optical fiber 107 on which the protective covering layer has been formed to a winding machine (not shown) via a turn pulley 109.
Currently, along with improvement in productivity or cost reduction of optical fibers, enlargement of an optical fiber preform and speeding up of fiber drawing speed (hereinafter referred to as drawing speed) are being pursued. The following phenomena occur with the speeding up of the drawing speed.                The length of the cooling device required for cooling a bare optical fiber increases.        The flow rate per unit time of the cooling gas which flows alongside the bare optical fiber which comes out from the heating furnace increases.        The flow rate per unit time of the cooling gas which flows out of the inside of the cooling device along with the bare optical fiber increases.        
From the above, the concentration of the cooling gas within the cooling device decreases with the speeding up in the drawing speed, and the cooling capacity of the cooling device declines. As a result, there is a case where cooling of the bare optical fiber within the cooling device becomes insufficient, and the external diameter (hereinafter referred to as coating diameter) of the protective covering layer becomes small, or there is a case where cooling of the bare optical fiber becomes unstable, and a fluctuation in the coating diameter increases. In order to solve the above problems, there is a demand for a cooling device with an improved cooling capacity and with a stable cooling capacity.
Generally, factors causing fluctuation of the coating diameter include, for example, a change in the temperature of the bare optical fiber when coating the coating resin, a change in the shear rate of the coating resin in a dice land within the coating device, and the like.
The change in the temperature of the bare optical fiber when covering the coating resin appears as a change in the cooling capacity of the cooling device when the bare optical fiber drawn out from the optical fiber preform is cooled until its temperature, due to the cooling gas, reaches a temperature at which it is capable of being coated, within a range of the drawing speed (the range of the drawing speed which fluctuates during manufacture of products, and center drawing speed±X (m/min)). This change in the cooling capacity has a great influence on the change in the coating diameter. Accordingly, it is desirable that the cooling device has the capacity to always cool the bare optical fiber stably, and the capacity capable of appropriately adjusting the temperature of the bare optical fiber within a drawing speed range which fluctuates during manufacture of products.
On the other hand, the shear rate of the coating resin in the dice land within the coating device mainly changes depending on a change in viscosity due to a change in the temperature of the coating resin or a change in the supply pressure of the coating resin into the coating device. However, within the drawing speed range, it may be considered that the influence that these changes have on the fluctuation in the coating diameter is small, and is almost non-existent.
As a technique for solving such problems, there is a method disclosed in Patent Literature 1 (Japanese Patent No. 4214389). The method described in Patent Literature 1 performs fiber drawing of an optical fiber preform 201, using a cooling device 211 including a fiber inlet portion and a fiber outlet portion of a bare optical fiber 204 as shown in FIG. 16, such that the pressure loss from a cooling gas introduction port to the fiber inlet portion of the bare optical fiber 204 is made lower than the pressure loss from the cooling gas introduction port to the fiber outlet portion of the bare optical fiber 204. In order to realize this method, a cooling method in which the fiber outlet portion of the bare optical fiber 204 is covered (sealed) with a resin coating device 205 is described in Patent Literature 1. Thereby, in practice, an outlet for the cooling gas introduced into the cooling device 211 becomes only the fiber inlet portion (upper portion) of the bare optical fiber 204 of the cooling device 211. Accordingly, the gas which is going to flow into the cooling device 211 along with the bare optical fiber 204 can be efficiently stripped off from the bare optical fiber 204, and the cooling efficiency of the cooling device 211 can be improved.
However, in the technique described in Patent Literature 1, in order to strip the gas accompanying the bare optical fiber off from the bare optical fiber, the flow of the cooling gas within the cooling device 211 is used as an upward flow by adjusting the pressure loss. In particular, in the technique described in Patent Literature 1, the fiber outlet portion of the bare optical fiber 204 of the cooling device 211 is covered with the resin coating device 205. Therefore, an inlet portion for ambient air into the cooling device 211 becomes only the fiber inlet portion (upper portion) of the bare optical fiber 204 of the cooling device 211, and the gas accompanying the bare optical fiber 204 can be efficiently stripped off from the bare optical fiber 204. Additionally, in the technique described in Patent Literature 1, a place where external gas is mixed into the cooling device 211 is only the upper portion of the cooling device 211. However, the upper portion of the cooling device 211 becomes a discharge port for the cooling gas from the inside of the cooling device 211. Therefore, the mixing of the external gas into the inside of the cooling device 211 is minimized, and the concentration of the cooling gas within the cooling device 211 becomes very high. As a result, the amount of the cooling gas used can be reduced. Moreover, the concentration of the cooling gas within the cooling device 211 becomes very high, and the heat exchange between the bare optical fiber 204 and the cooling gas and the heat exchange between the cooling gas and the cooling device 211 are efficiently performed.
However, the technique described in Patent Literature 1 has a problem that the cooling capacity of the cooling device 211 improves excessively, and the responsiveness of the cooling capacity to a change in the flow rate of the cooling gas becomes too agile. In the fiber drawing step of the optical fiber preform 210, exchange (flow of the cooling gas) of the cooling gas within the cooling device 211 may become non-uniform due to a disturbance. Here, the term “disturbance” means a fluctuation in the drawing speed (for example, a change of 60 m/min2 or more) accompanying an instantaneous change in the external diameter (for example, an external diameter change of a standard external diameter (generally 125 μm) of ±1 μm or more caused within several seconds) of the bare optical fiber 204 resulting from air bubbles or foreign matter mixed into the optical fiber preform 201, a fluctuation in the drawing speed (for example, a change of 30 m/min2 or more) accompanying a change in the external diameter of the optical fiber preform 201 (particularly, a change in the external diameter resulting from the portions of the tip portions and end portion of the optical fiber preform 201 where the mean external diameter of the optical fiber preform 201 changes by ±1 μm or more), and a temperature change (temperature change of, for example cooling water or inner wall, of the cooling device until the drawing speed becomes a normal drawing speed from the start of fiber drawing) accompanying a change over time of the cooling device. Since the responsiveness of the cooling capacity of the cooling device 211 is too agile in the technique described in Patent Literature 1, a problem occurs in that the cooling capacity of the cooling device 211 becomes unstable due to the flow of the non-uniform cooling gas generated by this disturbance.
In the technique described in Patent Literature 1, even in a case where the cooling gas flow rate has changed by a very small amount to adjust the cooling capacity according to the drawing speed fluctuation when the drawing speed becomes normal (manufacture center drawing speed or normal drawing speed), a change in the cooling capacity becomes large. Due to the change in the cooling capacity, there is a change in not only the heat exchange between the bare optical fiber and the cooling gas but also in the heat exchange between the cooling gas and the cooling device. Particularly, in the technique described in Patent Literature 1, the influence of these changes is great. Therefore, it is difficult to keep the coating diameter constant by a feedback control (for example, PID control) of a constant value.
On the other hand, if PID setting values which make feedback control insensitive are used, the coating diameter cannot be kept constant. As a result, there is a problem in that the coating diameter fluctuation increases and the percentage of defective manufactured optical fibers increases.